Traditionally, the traffic in the telecommunication networks has essentially been composed of real time traffic with deterministic and low bandwidth requirements, whereas in computer communication networks, to a considerable degree, it has been composed of non-real time traffic with a bursty and varying need of bandwidth. The different characteristics of the traffic flows have led to different implementations of the networks. The present development on the Internet with multimedia services, new communication services etc. results in new types of traffic flows, which makes new demands on the networks which are to transport and switch these traffic flows.
As regards telecommunication, the extremely fast development of the optical transmission systems, where transmission at Tbit/s was achieved already 1996, has given hopes of providing switched all-optical networks. However, up to now it has turned out that these networks are still limited as regards both the number of nodes that can be passed and the number of simultaneous connections which can be supported. Above all, this is due to coherent crosstalk between wavelength channels, amplifier saturation in the optical amplifiers and intensified spontaneous emission.
These deficiencies imply that networks with optical systems on links which are connected by means of electronic switches are still the most common ones. In this type of network, each link may comprise approximately ten fibres. Each fibre may carry tens of wavelength channels which, in their turn, each can be divided into a large quantity of logical channels. The most common technique for division into logical channels in transport networks is today the synchronous technique SDH (Synchronous Digital Hierarchy) in Europe and SONET (Synchronous Optical NETwork) which is the American correspondence. A disadvantage of these synchronous techniques is that they require clock distribution and synchronisation between the nodes of the network. Another disadvantage is that the distribution of bandwidth between logical channels is essentially static over time.
As regards computer communication, new equipment has been developed based on the network protocol IP (Internet Protocol) on the Internet. Today, IP constitutes the most important technique in networks for handling multimedia traffic, but also the newer technique ATM (Asynchronous Transfer Mode) is used to a certain degree. Both IP and ATM have the advantage of supporting the handling of a large number of traffic flows and of supporting the handling of data streams of different size.
A critical part in the handling of data streams is, of course, the switching or routing in the nodes. This part of the handling consists of two parts, on the one hand, the node must connect the traffic coming through the inputs to the correct output and, on the other hand, the traffic has to be multiplexed on the outputs.
In all-optical switches and in SDH and SONET switches both connection and multiplexing usually occur by means of so-called cross-connect switches which are adjusted in advance on the basis of a dimensioning which implies that contention on the outputs is avoided. This may take place since the bandwidth of the different channels in them are deterministic. Naturally, the disadvantage is that these channels are not suitable as regards traffic which has varying bandwidth requirements.
In IP routers and ATM switches connection and multiplexing may occur in real time. In both these cases, contention for bandwidth on the outputs may appear when packets coming from different inputs at the same point of time are to exit on the same output. This is usually solved by means of buffers and central processing units. However, these solutions require complicated equipment, and problems with the throughput arise at high data transfer rates and a plurality of inputs and outputs.